Merck (NYSE:MRK) stock could move up or down this week after the pharmaceutical company reports earnings on Wednesday. Here's a look at what to expect and the biggest divers of the price for Merck stock.

Currency hurts
For drugmakers that report in dollars, the strengthening of the dollar against other currencies hurts revenue and potentially earnings.

After the Venezuelan government devalued its currency in February, Merck said the move would hurt Merck's earnings by $0.05 per share this quarter and about $0.02 per share over the rest of the year.

Merck isn't the only drugmaker with lower earnings because of the currency change. Johnson & Johnson (NYSE:JNJ), for instance, reported a $0.04 hit to its earnings in the first quarter when it reported earlier this month. The diversified drugmaker incorporated the loss into earnings rather than treating it as a special item, and it was able to make up for the loss through other operating profits.

It doesn't look like the currency hit will hurt Merck stock, either. When announcing the expected loss, Merck didn't change its full-year earnings guidance.

Making up for Singulair
Sales of Merck's Singulair are going to drop substantially after generic versions of the allergy and asthma drug hit the market last year. But that's not going to affect the price of Merck stock. Investors know it's coming; the drop in sales is already priced in.

Instead, investors should focus on Merck's other drugs and how much of the gap they can make up.

The obvious savior should be Merck's Januvia diabetes franchise. In addition to Januvia, there's Janumet -- a combination of Januvia and a generic diabetes drug metformin, an extended release version of the combination called Janumet XR -- and Juvisync, which combines Januvia and another generic diabetes drug simvastatin.

The franchise faces ever-increasing competition from the likes of Bristol-Myers Squibb (NYSE:BMY) and AstraZeneca's (NYSE:AZN) Onglyza and Bydureon. Johnson & Johnson also has a new diabetes drug called Invokana, although that won't have an impact this quarter since it was just approved at the end of March. Investors shouldn't be too worried, though; doctors have little reason to switch from medications they're comfortable with, as witnessed by the rather slow starts of Onglyza and Bydureon. Last year, sales of the combined Januvia franchise increased 23%. With sales of $5.7 billion, that's a substantial boost to revenue.

The other product that could drive Merck stock higher is Gardasil. The pharma's HPV vaccine has had a bit of a resurgence, up 61% last quarter, thanks to a push to vaccinate males in addition to females it was originally approved for. The vaccine could certainly help drive revenue higher for a couple more quarters, but don't expect monster growth to last forever; once the push ends, we'll move into steady sales dictated by the population growth.

Buy before earnings?
It's a pharma. A good quarter might send Merck stock up a couple of percentage points. If Merck misses its adjusted earnings, perhaps we see a slight move downward. Either way, it's not going to matter all that much to the long-term performance of Merck stock.

That is why you're buying Merck stock, isn't it? If you own it, there seems to be little reason to sell ahead of earnings, and if Merck stock is on your watchlist, you might as well wait to see the first-quarter results to see if you'd like to be a shareholder.